Method for controlling compaction

ABSTRACT

A method of controlling compaction including obtaining a plurality of sets of process conditions for a plurality of glass ribbons, measuring a compaction value for a glass sheet cut from each glass ribbon of the plurality of glass ribbons, correlating the compaction to the process conditions. The method further includes selecting a predetermined cooling curve including a plurality of cooling rates, modifying the cooling curve by varying cooling rates of the plurality of cooling rates, calculating a predicted compaction value for a glass sheet cut from a glass ribbon drawn using the modified cooling curve, and repeating the modification and predicting until compaction is minimized.

CROSS-REFERENCE TO RELATED APPLICATIONS

The application claims the benefit of priority under 35 U.S.C. § 371 ofInternational Application No. PCT/US18/37596, filed on Jun. 14, 2018,which claims the benefit of priority of U.S. Provisional ApplicationSer. No. 62/519,347 filed on Jun. 14, 2017 the contents of which arerelied upon and incorporated herein by reference in their entirety as iffully set forth below.

BACKGROUND Field

The present disclosure relates generally to methods for reducingdistortion of glass substrates, and more particularly, to minimizingcompaction thereof.

Technical Background

Glass display panels in the form of liquid crystal displays (LCDs) areused in an increasing variety of applications—from hand-held phones tocomputer monitors to television displays. These applications requireglass sheets with pristine, defect-free surfaces. LCDs are comprised ofthin sheets of glass sealed together to form an envelope. It is highlydesirable that the dimensions of glass sheets comprising these displaysdo not vary when thermally cycled to maintain proper registration, oralignment, between elements comprising the LCD.

Typically, LCDs are of the amorphous silicon (a-Si) thin film transistor(TFT) or polycrystalline-silicon (ρ-Si or poly-Si) TFT type. Poly-Si hasa much higher drive current and electron mobility, thereby decreasingthe response time of the pixels. Further, it is possible, using ρ-Siprocessing, to build the display drive circuitry directly on the glasssubstrate. By contrast, α-Si requires discrete driver chips that must beattached to the display periphery utilizing integrated circuit packagingtechniques.

However, the manufacture of ρ-Si TFTs requires higher processingtemperatures than α-Si TFTs, thereby increasing the risk of glassshrinkage (compaction) during processing. In addition, continuingimprovements in display resolution by increasing pixel density has alsorequired commensurate improvements in glass thermal stability to ensureproper alignment of display components during manufacture, such asduring deposition of thin film transistors on the glass substrate.

Glass manufacturers have often subjected glass substrates to a heattreatment prior to shipping the glass substrates to customers so thatthe substrates do not shrink, or shrink very little, when subjected tolater thermal cycling in a customer process. Such heat treatments areknown as “pre-shrinking” or “pre-compacting.” High temperatureprocessing, such as required by ρ-Si TFTs, may require long heattreatment times for the glass substrate to ensure low compaction, e.g.,5 hours at 600° C. Moreover, such heat treatments involve furtherhandling of the glass substrates, increasing the chances of damage tothe surfaces of the substrates as well as increasing overallmanufacturing costs.

Quantitatively, compaction is the change in length per unit lengthexhibited by a glass substrate in a plane of the substrate as a resultof subtle changes in glass structure produced by thermal cycling (i.e.,compaction is strain resulting from the thermal history of the glass andis closely associated with the fictive temperature of the glass).Compaction can be determined physically by placing marks on a glasssubstrate and measuring the initial distance between the marks. Thesubstrate is then subjected to a predetermined heat treatment andreturned to room temperature. The distance between the marks is thenre-measured. Compaction in parts-per-million (ppm) is then given by:compaction=10⁶. (distance before heat treatment—distance after heattreatment)/(distance before heat treatment).

Because compaction is of concern to LCD panel manufacturers, in thepast, as molten glass flow increases were made to increase output, themanufacturing process was linearly scaled to allow sufficient timeduring cooling to maintain the same compaction of the finishedsubstrates as existed before the flow increase. Although this approachworks to an extent, it has the drawback that it can require an increaseddistance between the forming body and the location where substrates areseparated from the glass ribbon. These longer distances take upadditional manufacturing space and capital to implement. Indeed, due tothe physical constraints of existing facilities, increasing the drawdistance to reduce compaction can limit the maximum flow available to agiven glass forming installation. Moreover, ensuring the appropriatecompaction often required significant experimentation to obtain theappropriate process conditions. It would be beneficial to be able topredict the compaction obtained by selected process conditions.

SUMMARY

In accordance with an embodiment of the present disclosure, a method ofcontrolling compaction is disclosed, comprising:

-   -   a) measuring compaction for a plurality of glass sheets cut from        a plurality of glass ribbons formed with different cooling        rates;    -   b) correlating the measured compaction with the cooling rates of        step a) to obtain a plurality of regression coefficients        corresponding to a plurality of temperatures; c) selecting a        predetermined cooling curve, the predetermined cooling curve        comprising a plurality of predetermined cooling rates at the        corresponding plurality of temperatures of step (b);    -   d) using the plurality of regression coefficients and the        plurality of predetermined cooling rates to calculate a        predicted compaction resulting from the predetermined cooling        curve;    -   e) modifying the predetermined cooling rates to minimize the        predicted compaction and obtain target cooling rates;    -   f) drawing a subsequent glass ribbon using the target cooling        rates.

The method may further comprise substituting the modified cooling ratesof step e) for the predetermined cooling rates of step d), and repeatingsteps d) and e) to obtain new target cooling rates prior to step f).This iterative process may be repeated as many times as necessary untila target cooling rate is obtain that minimizes compaction.

Step b) may comprise a linear regression, such as of the form

${{\begin{bmatrix}k \\\vdots \\k\end{bmatrix} + {\begin{bmatrix}q_{1}^{1} & \ldots & q_{1}^{n} \\\vdots & \ddots & \vdots \\q_{i}^{1} & \ldots & q_{i}^{n}\end{bmatrix} \times \begin{bmatrix}b_{1} \\\vdots \\b_{n}\end{bmatrix}}} = \begin{bmatrix}C_{1} \\\vdots \\C_{i}\end{bmatrix}},$where q represents cooling rate in ° C./second, b represents theregression coefficients, C represents compaction in parts per million, irepresents a total number of data sets, n represents a total number ofthe regression coefficients, and k represents an intercept of theregression.

The predicted compaction value can be calculated from the equation,Compaction=k+Σ _(n=1) ^(n=m)(b _(n) ×q _(n)), wherek is an intercept of the regression, m represents a number oftemperature increments over a temperature range from x° C. to y° C., nrepresents a number of temperature increments, b represents theregression coefficients and q represents cooling rate.

In some embodiments, x is equal to 450° C. and y is equal to 900° C.

In another embodiment, a method of controlling compaction is described,comprising:

-   -   a) drawing a glass ribbon from a forming body at a first draw        rate;    -   b) measuring temperature along a centerline of the glass ribbon        at a plurality of distances from a bottom edge of the forming        body;    -   c) calculating cooling rates for the glass ribbon at the        plurality of distances based on the measured temperatures;    -   d) measuring compaction of a glass sheet cut from the glass        ribbon;    -   e) repeating steps a) through d) for a plurality of glass        ribbons drawn at a plurality of draw rates to obtain a plurality        of measured compaction values;    -   f) correlating the plurality of measured compaction values with        the cooling rates at the plurality of distances for the        plurality of glass ribbons to obtain a plurality of regression        coefficients corresponding to a plurality of temperatures in a        predetermined temperature range;    -   g) selecting a predetermined cooling curve, the predetermined        cooling curve comprising a predetermined cooling rate at each        corresponding temperature of the plurality of temperatures;    -   h) using the plurality of regression coefficients and the        plurality of predetermined cooling rates to obtain a predicted        compaction value at a predetermined draw rate;    -   i) modifying the predetermined cooling rates to minimize the        predicted compaction value at the predetermined draw rate and        obtain target cooling rates;    -   j) drawing a subsequent glass ribbon using the target cooling        rates.

The method may further comprise substituting the modified cooling ratesof step i) for the predetermined cooling rates of step h), and repeatingsteps h) and i) to obtain new target cooling rates prior to step j).This iterative process may be repeated as many times as necessary untila target cooling rate is obtain that minimizes compaction.

Step f) may comprise, a linear regression, such as a system of linearequations of the form

${{\begin{bmatrix}k \\\vdots \\k\end{bmatrix} + {\begin{bmatrix}q_{1}^{1} & \ldots & q_{1}^{n} \\\vdots & \ddots & \vdots \\q_{i}^{1} & \ldots & q_{i}^{n}\end{bmatrix} \times \begin{bmatrix}b_{1} \\\vdots \\b_{n}\end{bmatrix}}} = \begin{bmatrix}C_{1} \\\vdots \\C_{i}\end{bmatrix}},$where q represents cooling rate in ° C./second, b represents theregression coefficients, C represents compaction value in parts permillion, i represents a total number of data sets, n represents a totalnumber of temperature increments, and k represents an intercept of theregression.

The predicted compaction value can be calculated as,Compaction=k+Σ _(n=1) ^(n=m)(b _(n) ×q _(n)), wherek is an intercept of the regression, m represents a number oftemperature increments over a temperature range from x° C. to y° C., nrepresents temperature increments, b is a regression coefficient and qis cooling rate.

In some embodiments, x is equal to 450° C. and y is equal to 900° C.

Additional features and advantages of the embodiments disclosed hereinwill be set forth in the detailed description that follows, and in partwill be readily apparent to those skilled in the art from thatdescription or recognized by practicing the invention as describedherein, including the detailed description which follows, the claims, aswell as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description present embodiments intended toprovide an overview or framework for understanding the nature andcharacter of the embodiments disclosed herein. The accompanying drawingsare included to provide further understanding, and are incorporated intoand constitute a part of this specification. The drawings illustratevarious embodiments of the disclosure, and together with the descriptionserve to explain the principles and operations thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot graphically showing several variations of exemplarycooling curves;

FIG. 2 is a schematic view of an exemplary fusion down draw apparatus;

FIG. 3 is a plot of compaction as a function of various sets of drawconditions labeled as A through L;

FIG. 4 is a plot of regression coefficients obtained by correlating thedraw conditions and compaction of FIG. 3 ;

FIG. 5 is a plot graphically illustrating several target cooling curves(temperature as a function of distance from the forming body) producedusing the regression coefficients of FIG. 4 ;

FIG. 6 is a plot of measured compaction obtained from the cooling curvesof FIG. 5 .

DETAILED DESCRIPTION

Reference will now be made in detail to embodiments of the presentdisclosure, examples of which are illustrated in the accompanyingdrawings. Whenever possible, the same reference numerals will be usedthroughout the drawings to refer to the same or like parts. However,this disclosure may be embodied in many different forms and should notbe construed as limited to the embodiments set forth herein.

Ranges can be expressed herein as from “about” one particular value,and/or to “about” another particular value. When such a range isexpressed, another embodiment includes from the one particular valueand/or to the other particular value. Similarly, when values areexpressed as approximations, by use of the antecedent “about,” it willbe understood that the particular value forms another embodiment. Itwill be further understood that the endpoints of each of the ranges aresignificant both in relation to the other endpoint, and independently ofthe other endpoint.

Directional terms as used herein—for example up, down, right, left,front, back, top, bottom—are made only with reference to the figures asdrawn and are not intended to imply absolute orientation.

Unless otherwise expressly stated, it is in no way intended that anymethod set forth herein be construed as requiring that its steps beperformed in a specific order, nor that with any apparatus, specificorientations be required. Accordingly, where a method claim does notactually recite an order to be followed by its steps, or that anyapparatus claim does not actually recite an order or orientation toindividual components, or it is not otherwise specifically stated in theclaims or description that the steps are to be limited to a specificorder, or that a specific order or orientation to components of anapparatus is not recited, it is in no way intended that an order ororientation be inferred, in any respect. This holds for any possiblenon-express basis for interpretation, including: matters of logic withrespect to arrangement of steps, operational flow, order of components,or orientation of components; plain meaning derived from grammaticalorganization or punctuation, and; the number or type of embodimentsdescribed in the specification.

As used herein, the singular forms “a,” “an” and “the” include pluralreferences unless the context clearly dictates otherwise. Thus, forexample, reference to “a” component includes aspects having two or moresuch components, unless the context clearly indicates otherwise.

As used herein, “molten glass” shall be construed to mean a moltenmaterial which, upon cooling, can enter a glassy state. The term moltenglass is used synonymously with the term “melt”. The molten glass mayform, for example, a majority silicate glass, although the presentdisclosure is not so limited.

As used herein, the term “cooling curve” shall denote temperature as afunction of distance, or alternatively as a function of time. Typically,distance is denoted relative to a bottom edge of a forming body fromwhich the ribbon of molten glass is drawn. It should be recognized thattime is directly relatable to distance given a known draw speed. Acooling curve may comprise one or more linear (constant) cooling rates,one or more nonlinear cooling rates, or a combination of linear andnonlinear cooling rates. For example, FIG. 1 depicts an exemplarycooling curve 8 a shown plotted as temperature as a function of distancebelow the forming body. Cooling curve 8 a comprises a single, linearcooling rate, whereas cooling curve 8 b depicts a plurality of linearcooling rates (segments). Curve 8 c comprises a nonlinear cooling curve.Cooling rates are determined as the slope of a tangent to the curve, orsegment, at the point (e.g., temperature) of interest. It should beunderstood that the cooling curves shown in FIG. 1 are merely exemplaryand presented for illustration, not limitation.

The manufacture of glass sheets by a draw process, for example a downdraw process such as a fusion down draw process, requires carefulcontrol of temperature. The impact of temperature can be most acuteduring the forming process, where a thin glass ribbon, in many casesless than a millimeter (mm) or less in thickness and in some instancesin excess of 3 meters wide, is drawn from a forming body through freespace supported principally by its edges. For example, a thickness ofthe glass ribbon at the centerline of the glass ribbon may be equal toor less than about 1 mm, such as equal to or less than about 0.7 mm,equal to or less than about 0.5 mm, equal to or less than about 0.3 mmand in some embodiments, equal to or less than about 0.1 mm.

By way of example, FIG. 2 illustrates an exemplary fusion glassmanufacturing apparatus 10. In some embodiments, the glass manufacturingapparatus 10 can comprise a glass melting furnace 12 that can include amelting vessel 14. In addition to melting vessel 14, glass meltingfurnace 12 can optionally include one or more additional components suchas heating elements (e.g., combustion burners and/or electrodes)configured to heat raw material and convert the raw material into moltenglass. For example, melting furnace 14 may be an electrically-boostedmelting vessel, wherein energy is added to the raw material through bothcombustion burners and by direct heating, wherein an electric current ispassed through the raw material, and thereby adding energy via Jouleheating of the raw material. As used herein, an electrically-boostedmelting vessel is a melting vessel that obtains heat energy from bothJoule heating and above-surface combustion heating, and the amount ofenergy imparted to the raw material and/or melt via Joule heating isequal to or greater than about 20%. As used herein, anelectrically-boosted melting vessel does not include submergedcombustion processes. In some embodiments, the heat energy added to themolten material by Joule heating (X) compared to the total heat energyadded to the molten material via both above-surface combustion burners(Y) and Joule heating can be in a range from about 20% to about 80%. Forexample, the ratio X:Y of heat energy added to the molten material viaJoule heating compared to above-surface combustion burners may be20%:80%, 30%:70%, 40%:60%, 50%:50%, 60%:40%, 70%:30% or even 80%:20%,although in further embodiments other ratios may be used.

In further embodiments, glass melting furnace 12 may include thermalmanagement devices (e.g., insulation components) that reduce heat lossfrom the melting vessel. In still further embodiments, glass meltingfurnace 12 may include electronic devices and/or electromechanicaldevices that facilitate melting of the raw material into a glass melt.Still further, glass melting furnace 12 may include support structures(e.g., support chassis, support member, etc.) or other components.

Glass melting vessel 14 is typically formed from a refractory material,such as a refractory ceramic material, for example a refractory ceramicmaterial comprising alumina or zirconia, although the refractory ceramicmaterial may comprise other refractory materials, such as yttrium (e.g.,yttria, yttria stabilized zirconia, yttrium phosphate), zircon (ZrSiO4)or alumina-zirconia-silica or even chrome oxide, used eitheralternatively or in any combination. In some examples, glass meltingvessel 14 may be constructed from refractory ceramic bricks.

In some embodiments, melting furnace 12 may be incorporated as acomponent of a glass manufacturing apparatus configured to fabricate aglass article, for example a glass ribbon of an indeterminate length,although in further embodiments, the glass manufacturing apparatus maybe configured to form other glass articles without limitation, such asglass rods, glass tubes, glass envelopes (for example, glass envelopesfor lighting devices, e.g., light bulbs) and glass lenses, although manyother glass articles are contemplated. In some examples, the meltingfurnace may be incorporated as a component of a glass manufacturingapparatus comprising a slot draw apparatus, a float bath apparatus, adown draw apparatus (e.g., a fusion down draw apparatus), an up drawapparatus, a pressing apparatus, a rolling apparatus, a tube drawingapparatus or any other glass manufacturing apparatus that would benefitfrom the present disclosure. By way of example, FIG. 2 schematicallyillustrates glass melting furnace 12 as a component of a fusion downdraw glass manufacturing apparatus 10 for fusion drawing a glass ribbonfor subsequent processing into individual glass sheets or rolling theglass ribbon onto a spool.

Glass manufacturing apparatus 10 (e.g., fusion down draw apparatus 10)can optionally include an upstream glass manufacturing apparatus 16positioned upstream relative to glass melting vessel 14. In someexamples, a portion of, or the entire upstream glass manufacturingapparatus 16, may be incorporated as part of the glass melting furnace12.

As shown in the embodiment illustrated in FIG. 2 , the upstream glassmanufacturing apparatus 16 can include a raw material storage bin 18, araw material delivery device 20 and a motor 22 connected to the rawmaterial delivery device. Storage bin 18 may be configured to store aquantity of raw material 24 that can be fed into melting vessel 14 ofglass melting furnace 12 through one or more feed ports, as indicated byarrow 26. Raw material 24 typically comprises one or more glass formingmetal oxides and one or more modifying agents. In some examples, rawmaterial delivery device 20 can be powered by motor 22 such that rawmaterial delivery device 20 delivers a predetermined amount of rawmaterial 24 from the storage bin 18 to melting vessel 14. In furtherexamples, motor 22 can power raw material delivery device 20 tointroduce raw material 24 at a controlled rate based on a level ofmolten glass sensed downstream from melting vessel 14 relative to a flowdirection of the molten glass. Raw material 24 within melting vessel 14can thereafter be heated to form molten glass 28. Typically, in aninitial melting step, raw material is added to the melting vessel asparticulate, for example as comprising various “sands”. Raw material mayalso include scrap glass (i.e. cullet) from previous melting and/orforming operations. Combustion burners are typically used to begin themelting process. In an electrically boosted melting process, once theelectrical resistance of the raw material is sufficiently reduced (e.g.,when the raw materials begin liquefying), electric boost is begun bydeveloping an electric potential between electrodes positioned incontact with the raw materials, thereby establishing an electric currentthrough the raw material, the raw material typically entering, or in, amolten state at this time.

Glass manufacturing apparatus 10 can also optionally include adownstream glass manufacturing apparatus 30 positioned downstream ofglass melting furnace 12 relative to a flow direction of the moltenglass 28. In some examples, a portion of downstream glass manufacturingapparatus 30 may be incorporated as part of glass melting furnace 12.However, in some instances, first connecting conduit 32 discussed below,or other portions of the downstream glass manufacturing apparatus 30,may be incorporated as part of the glass melting furnace 12. Elements ofthe downstream glass manufacturing apparatus, including first connectingconduit 32, may be formed from a precious metal. Suitable preciousmetals include platinum group metals selected from the group of metalsconsisting of platinum, iridium, rhodium, osmium, ruthenium andpalladium, or alloys thereof. For example, downstream components of theglass manufacturing apparatus may be formed from a platinum-rhodiumalloy including from about 70% to about 90% by weight platinum and about10% to about 30% by weight rhodium. However, other suitable metals caninclude molybdenum, rhenium, tantalum, titanium, tungsten and alloysthereof.

Downstream glass manufacturing apparatus 30 can include a firstconditioning (i.e. processing) vessel, such as fining vessel 34, locateddownstream from melting vessel 14 and coupled to melting vessel 14 byway of the above-referenced first connecting conduit 32. In someexamples, molten glass 28 may be gravity fed from melting vessel 14 tofining vessel 34 by way of first connecting conduit 32. For instance,gravity may drive molten glass 28 through an interior pathway of firstconnecting conduit 32 from melting vessel 14 to fining vessel 34. Itshould be understood, however, that other conditioning vessels may bepositioned downstream of melting vessel 14, for example between meltingvessel 14 and fining vessel 34. In some embodiments, a conditioningvessel may be employed between the melting vessel and the fining vesselwherein molten glass from a primary melting vessel is further heated ina secondary vessel to continue the melting process, or cooled to atemperature lower than the temperature of the molten glass in theprimary melting vessel before entering the fining vessel.

As described previously, bubbles may be removed from molten glass 28 byvarious techniques. For example, raw material 24 may include multivalentcompounds (i.e. fining agents) such as tin oxide that, when heated,undergo a chemical reduction reaction and release oxygen. Other suitablefining agents include without limitation arsenic, antimony, iron andcerium, although as noted previously, the use of arsenic and antimonymay be discouraged for environmental reasons in some applications.Fining vessel 34 is heated to a temperature greater than the meltingvessel temperature, thereby heating the fining agent. Oxygen bubblesproduced by the temperature-induced chemical reduction of one or morefining agents included in the melt rise through the molten glass withinthe fining vessel, wherein gases in the molten glass produced in themelting furnace can coalesce or diffuse into the oxygen bubbles producedby the fining agent. The enlarged gas bubbles with increased buoyancycan then rise to a free surface of the molten glass within the finingvessel and thereafter be vented out of the fining vessel. The oxygenbubbles can further induce mechanical mixing of the molten glass in thefining vessel as they rise through the molten glass.

The downstream glass manufacturing apparatus 30 can further includeanother conditioning vessel, such as a mixing apparatus 36, for examplea stirring vessel, for mixing the molten glass that flows downstreamfrom fining vessel 34. Mixing apparatus 36 can be used to provide ahomogenous glass melt composition, thereby reducing chemical or thermalinhomogeneities that may otherwise exist within the fined molten glassexiting the fining vessel. As shown, fining vessel 34 may be coupled tomixing apparatus 36 by way of a second connecting conduit 38. In someembodiments, molten glass 28 may be gravity fed from the fining vessel34 to mixing apparatus 36 by way of second connecting conduit 38. Forinstance, gravity may drive molten glass 28 through an interior pathwayof second connecting conduit 38 from fining vessel 34 to mixingapparatus 36. Typically, the molten glass within the mixing apparatusincludes a free surface, with a free volume extending between the freesurface and a top of the mixing apparatus. It should be noted that whilemixing apparatus 36 is shown downstream of fining vessel 34 relative toa flow direction of the molten glass, mixing apparatus 36 may bepositioned upstream from fining vessel 34 in other embodiments. In someembodiments, downstream glass manufacturing apparatus 30 may includemultiple mixing apparatus, for example a mixing apparatus upstream fromfining vessel 34 and a mixing apparatus downstream from fining vessel34. These multiple mixing apparatus may be of the same design, or theymay be of a different design from one another. In some embodiments, oneor more of the vessels and/or conduits may include static mixing vanespositioned therein to promote mixing and subsequent homogenization ofthe molten material.

Downstream glass manufacturing apparatus 30 can further include anotherconditioning vessel such as delivery vessel 40 that may be locateddownstream from mixing apparatus 36. Delivery vessel 40 may conditionmolten glass 28 to be fed into a downstream forming device. Forinstance, delivery vessel 40 can act as an accumulator and/or flowcontroller to adjust and provide a consistent flow of molten glass 28 toforming body 42 by way of exit conduit 44. The molten glass withindelivery vessel 40 can, in some embodiments, include a free surface,wherein a free volume extends upward from the free surface to a top ofthe delivery vessel. As shown, mixing apparatus 36 may be coupled todelivery vessel 40 by way of third connecting conduit 46. In someexamples, molten glass 28 may be gravity fed from mixing apparatus 36 todelivery vessel 40 by way of third connecting conduit 46. For instance,gravity may drive molten glass 28 through an interior pathway of thirdconnecting conduit 46 from mixing apparatus 36 to delivery vessel 40.

Downstream glass manufacturing apparatus 30 can further include formingapparatus 48 comprising the above-referenced forming body 42, includinginlet conduit 50. Exit conduit 44 can be positioned to deliver moltenglass 28 from delivery vessel 40 to inlet conduit 50 of formingapparatus 48. Forming body 42 in a fusion down draw glass makingapparatus can comprise a trough 52 positioned in an upper surface of theforming body and converging forming surfaces 54 (only one surface shown)that converge in a draw direction along a bottom edge (root) 56 of theforming body. Molten glass delivered to the forming body trough viadelivery vessel 40, exit conduit 44 and inlet conduit 50 overflows thewalls of the trough and descends along the converging forming surfaces54 as separate flows of molten glass. It should be noted that the moltenglass within the forming body trough comprises a free surface, and afree volume extends from the free surface of the molten glass to the topof an enclosure within which the forming body is positioned. Theseparate flows of molten glass join below and along the root to producea single ribbon of molten glass 58 that is drawn in a draw direction 60from root 56 by applying a downward tension to the glass ribbon, such asby gravity, edge rolls and pulling rolls (not shown), to control thedimensions of the glass ribbon as the molten glass cools and a viscosityof the material increases. Accordingly, glass ribbon 58 goes through avisco-elastic transition and acquires mechanical properties that giveglass ribbon 58 stable dimensional characteristics. Glass ribbon 58 mayin some embodiments be separated into individual glass sheets 62 by aglass separation apparatus (not shown) in an elastic region of the glassribbon, while in further embodiments, the glass ribbon may be wound ontospools and stored for further processing.

It should be readily apparent that even small temperature variationsalong and across a glass ribbon drawn through free space can result inresidual stress that can warp the glass ribbon, and the glass sheet cuttherefrom. It is also important to note that the ribbon typically leavesthe forming body at a temperature in excess of 1000° C., but must becooled to a temperature less than only several hundred degrees in a veryshort distance since the ribbon is typically drawn in a verticaldownward direction and available vertical distance is often limited bypractical considerations.

It should be further recognized that the capital expenditure required toassemble and operate a down draw glass making process, particularly forthe production of optical quality glass, can be quite large. Forexample, with the exception of melting vessel 14 and forming vessel 42,which are usually constructed from a non-metallic refractory material,the remainder of the apparatus, including intervening vessels andconduits that carry the molten glass from the melting vessel to theforming body, are formed from one or more precious metals. Accordingly,changes in production volume demand beyond the existing apparatus limitsare difficult and expensive to address.

One aspect of increased molten glass flow rate is the increased heatload imposed on the manufacturing apparatus, which can upset the thermalbalance of the process, from melting furnace to below the forming body.That is, as the flow rate increases, methods must be found to adequatelycool the molten glass to achieve the appropriate molten glass viscosityand forming characteristics, particularly within the draw portion of theprocess.

To maintain a reasonably consistent environment, both the forming bodyand the free space region through which the molten glass ribbon is drawnas it transitions from a viscous liquid to an elastic solid is containedwithin structures that separate the molten glass ribbon from thesurrounding environment. More particularly, the free space volumethrough which the glass ribbon is drawn is surrounded on at least foursides by a housing positioned below the forming body: a collection ofconnected walls and refractory insulation that form a shroud oropen-ended box, or, in effect, a vertically oriented tunnel. Heatingand/or cooling devices (not shown) necessary to control the temperatureof the ribbon (e.g., cool the ribbon) are positioned within the housingalong draw direction 60 and in a width (lateral) direction orthogonal tothe draw direction. Such heating and cooling devices can compriseelectrical heating elements, cooling coils through which a coolant isflowed, or other devices, such as lasers configured to control thetemperature of the ribbon. Such devices are well known to those of skillin the art and are not further described here.

As noted, it is particularly beneficial during the drawing process tomaintain a well-controlled temperature regime at the temperature rangeover which the viscous ribbon transitions to an elastic solid. Moreover,it is desirable to cool the molten glass ribbon as quickly as possibleafter the molten glass leaves the forming body to maximize the availablespace needed to anneal the glass ribbon. Increasing the anneal periodallows time for the glass to undergo additional relaxation.

Relaxation behavior is important for many glass products. For example,liquid crystal display glass is subjected to thermal treatments duringdeposition of thin film transistors on the glass substrate. Relaxationof the glass during these heat treatment cycles can lead to compaction,i.e., a permanent change in the dimensions of the glass brought about bya densification of the glass. The quality of glass for precisionproducts, such as LCD manufacture, depends on obtaining a uniformthermal history throughout the glass; any uneven relaxation effect willlead to a deterioration of the quality of the final product throughoptical inhomogeneity (e.g., birefringence). When glass sheets are usedas substrates and subjected to elevated temperature during processing,glass relaxation can cause dimensional changes that impact subsequentmanufacturing processes. Minimal dimensional change during customerprocesses can be a key attribute of the glass. Thus, the ability toaccurately predict compaction under a specific set of process conditionswould be beneficial.

There are two important factors governing glass relaxation:thermodynamics and kinetics. Thermodynamically, glass is anon-equilibrium system that wants to relax. While the presence of athermodynamic driving force is a necessary condition for glassrelaxation, it is by itself insufficient. The glass must also havesufficient thermal energy and time to enable the kinetics of relaxation.Assuming isobaric conditions, the kinetics of the glass depend on threefactors: composition, temperature, and thermal history. The importanceof thermal history cannot be overstated, since the dynamic behavior oftwo glasses of the same composition and at the same temperature can varyby many orders of magnitude depending on the details of their thermalhistory.

The production of liquid crystal displays, for example active matrixliquid crystal display devices (AMLCDs), is complex, and the propertiesof the substrate glass are extremely important. First and foremost, theglass substrates used in the production of liquid crystal displaydevices need to have their physical dimensions tightly controlled.

In the liquid crystal display field, thin film transistors (TFTs) basedon poly-crystalline silicon are preferred for their ability to transportelectrons more effectively. Poly-crystalline based silicon transistors(ρ-Si) are characterized as having a higher mobility than those based onamorphous-silicon based transistors (α-Si). This allows the manufactureof smaller and faster transistors, which ultimately produces brighterand faster displays. One problem with ρ-Si based transistors is thattheir manufacture requires higher process temperatures than thoseemployed in the manufacture of α-Si transistors. Process temperaturescan range from about 450° C. to about 700° C. compared to peaktemperatures of about 350° C. typically employed in the manufacture ofα-Si transistors. At temperatures suitable for ρ-Si deposition, mostAMLLCD glass substrates undergo compaction. Compaction, also referred toas thermal stability or dimensional change, is an irreversibledimensional change (shrinkage) in the glass substrate due to changes inthe fictive temperature of the glass. “Fictive temperature” is a conceptused to indicate the structural state of a glass. Glass that is cooledquickly from a high temperature typically exhibits a higher fictivetemperature than an identical glass cooled from the same temperaturemore slowly because of the “frozen in” higher temperature structure.When a glass is held at an elevated temperature, the glass structure isallowed more time to relax toward the heat treatment temperaturestructure. Since the fictive temperature of the glass substrate used inLCD construction is almost always above the relevant heat treatmenttemperatures encountered during thin film transistor (TFT) processes,structural relaxation that occurs as a result of the high temperatureheat treatment causes a decrease in fictive temperature that causes anincrease in the glass density and a commensurate shrinkage (compaction).

It would be advantageous to minimize the propensity of the glass tocompact because compaction can create possible alignment issues duringthe display manufacturing process (e.g., during transistor deposition),which in turn may create performance issues in the finished display.

There are several approaches to minimize compaction in glass. Oneapproach is to thermally pretreat the glass to create a fictivetemperature similar to the temperature the glass will experience duringρ-Si TFT manufacture. There are several difficulties with this approach.First, the multiple heating steps employed during the ρ-Si TFTmanufacture create slightly different fictive temperatures in the glassthat cannot be fully compensated for by pretreatment. Second, thethermal stability of the glass becomes closely linked to the details ofthe particular ρ-Si TFT manufacture, which can mean differentpretreatments for different end-users. Finally, pretreatment adds toprocess cost and complexity.

Another approach involves slowing the cooling rate during manufacture.While such an approach has merits, some manufacturing techniques, suchas the fusion process, have only limited space available during thedrawing process to perform the slow cooling. Consequently, relativelyrapid quenching of the glass ribbon occurs, and a relatively hightemperature structure (high fictive temperature) is “frozen in”. Whilecontrolling the cooling rate(s) is possible with such a manufacturingprocess, development of optimal cooling rates that minimize compactioncan be difficult.

Historically, cooling rates for a given glass have been “borrowed” fromsimilar glasses manufactured under similar process conditions. That is,a cooling curve applied to one glass composition under one set ofprocess conditions may be applied to the manufacturing, for example, ofa another glass that is of a similar composition to the first glass as astarting point. Similarly, changes to process conditions, such as anincrease in throughput (e.g., glass flow rate), begins with thelower-flow cooling rates. In either case, optimization of this initialcooling curve, for example to minimize compaction for the new glassand/or process conditions, is undertaken on a “best guess” basis,wherein changes are made to the individual cooling rates comprising theinitial cooling curve using experience and, to a large degree, luck. Theresultant glass is tested, and if the attribute of choice, such ascompaction, is unsatisfactory, the cooling rates are modified. Theprocess is an iterative process played out in real time by the actualdrawing and testing of glass. It can be a lengthy endeavor and yet maynot result in the best cooling curve to deliver optimal compactionperformance.

It was long thought that the most beneficial cooling curve in thecontext of compaction comprised a fast cooling rate during the time theglass was in a generally low viscosity state (e.g., less than about 10¹⁰poise), after which the glass was cooled at a significantly slower ratein a viscosity region at or about the anneal point of the glass (theanneal point is defined as the temperature at which glass viscosity isequal to 10^(13.18) Poise). For example, U.S. Pat. No. 8,136,371describes a cooling regime where slow cooling is performed between theanneal point and 50° C. less than the anneal point. As discussed hereinbelow, the present inventors have discovered that the sensitivity ofcompaction to cooling rate can, unexpectedly, extend to temperatureswell below the anneal point. Thus, simply adjusting historical coolingrates with minimal guidance may not place the slow cooling regime in theappropriate temperature range to minimize compaction. It would bebeneficial to be able to predict compaction for a given set of processconditions and/or glass compositions to both ensure a maximum reductionin compaction and to reduce time spent searching for the appropriatecooling curve by trial and error.

Accordingly, a partial empirical method capable of predicting compactionis disclosed. In a first step, data from a plurality of processconditions is obtained. This data may be obtained from operational drawsduring normal, stable manufacturing, or from laboratory experiments. Thedata may include, for example, temperature of the glass ribbon along acenterline of the glass ribbon as a function of time and/or distancefrom the root of the forming body (since both are directly relatable),cooling rate (obtainable from the temperature, and time and/or distancefrom the root), and measured compaction for at least one glass sheet cutfrom each glass ribbon. Cooling rates can be calculated, for example, ata predetermined distance or temperature interval, for instance at every10° C., every 15° C., or any other suitable interval that provides thedesired measurement resolution. It should be evident in view of theforegoing that the temperature of the glass ribbon changes as a functionof distance from the forming body root, which has an equivalency intime, and therefore the interval can be selected based on time, location(distance) or temperature. In most scenarios, it may be acceptable toeliminate cooling rates at temperatures below about 450° C. and aboveabout 900° C., since at glass temperatures below 450° C. and above 900°C. compaction can be assumed to not be affected by cooling rate. Thatis, at temperatures above about 900° C., relaxation of the ribbon can beassumed to be sufficiently rapid that compaction is of little concern,and at temperatures below about 450° C., the glass ribbon issufficiently cooled that compaction behavior is frozen in and thereforeunaffected by thermal treatment at such low temperatures. If desired,multiple glass sheets may be measured for compaction for each processcondition to improve confidence in the data. The data may be obtainedfrom multiple draw apparatus or draw conditions for a given glasscomposition.

Once the foregoing data has been collected and/or calculated, a systemof simple linear regression equations can be employed with cooling ratesas predictor variables and compaction as response variables, as shown inequation (1) below,

$\begin{matrix}{{{\begin{bmatrix}k \\\vdots \\k\end{bmatrix} + {\begin{bmatrix}q_{1}^{1} & \ldots & q_{1}^{n} \\\vdots & \ddots & \vdots \\q_{i}^{1} & \ldots & q_{i}^{n}\end{bmatrix} \times \begin{bmatrix}b_{1} \\\vdots \\b_{n}\end{bmatrix}}} = \begin{bmatrix}C_{1} \\\vdots \\C_{i}\end{bmatrix}},} & (1)\end{matrix}$where q represents cooling rate in ° C./second, b represents theregression coefficients, C represents compaction in parts per million(ppm), i represents the number of data sets (number of unique sets ofprocess conditions), n represents the number of regression coefficients,and k represents the regression intercept.

Since the number of data sets may not equal the number of regressioncoefficients, principal component regression techniques may be used tofind an appropriate set of coefficients. Alternatively, the coefficientcalculation can be regularized by defining a penalty parameter thatpenalizes the differences among coefficients. Such techniques are wellknown and not further described herein.

Once the regression coefficients have been calculated, for a giventarget cooling curve cooling rates within a predetermined temperaturerange from x° C. to y° C. can be obtained and a compaction valuecalculated using the regression coefficients in accordance with equation(2),Compaction=k+Σ _(n=1) ^(n=m)(b _(n) ×q _(n)),  (2)where the summation is carried out over the predetermined range from xto y. In some embodiments, the summation may extend over a finitetemperature range, for example over a range from 450° C. to 900° C.,however, a range from 450° C. to 900° C. is provided as an example only,and other temperature ranges may be employed as desired. For example, inembodiments, x can be equal to or greater than 450° C. and y can beequal to or less than 950° C.

Minimizing compaction then becomes an iterative calculation process,involving modification of the initial cooling curve and subsequentcalculation of a predicted compaction of the new, modified coolingcurve, and may be driven, in part, by knowledge obtained by examiningthe regression coefficients. This is illustrated by the followingexample.

Corning® Lotus™ glass was drawn as glass ribbons from different drawapparatus under different process conditions, including differentthicknesses, flow rates and cooling rates, and compaction values weremeasured for glass sheets cut from the various ribbons. In all, data for12 different process conditions was collected. Compaction for glassribbons from each set of process conditions was measured by placing aglass sheet cut from each ribbon on a table with fiducial markingsthereon, fixing the glass sheet such that the glass sheet could beremoved and then accurately replaced. Matching fiducial markings weremarked on the glass sheet. The glass sheet was removed and heated in anoven to a temperature of 590° C., and held at that temperature for atotal of 30 minutes. After cooling the glass sheet at the oven rate(natural rate of cooling with the oven power off) to room temperature,the glass sheet was replaced on the table in the original position, andthe distance between the fiducial markings on the glass sheet and thefiducial markings on the table were measured. The distance between thetwo sets of fiducial markings represented a compaction value expressedin ppm. Glass sheets for each process condition were measured forcompaction multiple times and averaged. Compaction for all 12 sets ofprocess conditions is plotted in FIG. 3 . The data show a significantspread in compaction values, with glass sheet formed at a thickness of0.7 mm thickness (Sample C) exhibiting the least compaction, and glasssheet formed with a thickness of 0.3 mm (Samples G and L) exhibiting thegreatest compaction.

The process data and compaction values for the foregoing samples was setto the form of equation (1), with n=46 (46 regression coefficients) andi=12 (12 data sets), and solved for the regression coefficients b. Thenumber of regression coefficients will vary depending on the selectedtemperature interval at which the cooling rate data is calculated. Inthis example, data was collected at intervals of 10° C. in a range from450° C. to 950° C., although other intervals and ranges can be used,depending on need and the resolution desired. A plot of the regressioncoefficients as a function of temperature is provided in FIG. 4 . Largernegative numbers represent greater impact on compaction. For reference,the anneal point of Corning Lotus glass is 810° C. Examination of FIG. 4reveals that cooling rates for this particular glass in a range fromabout 660° C. to about 850° C. are the most impactful relative tocompaction. More importantly, the greatest impact on compaction occursat a temperature of about 710° C., fully 100° C. less than the annealpoint. It should be emphasized that these results pertain to the testedglass and that specific values may differ for other glass compositions.However, it remains that an examination of the regression coefficientsprovides insight into compaction behavior useful for guidingmodification of a cooling curve, and that compaction can be meaningfullyimpacted at temperatures well below the anneal point.

FIG. 5 depicts four cooling curves related to the foregoing example.Curve B represents a baseline (historical cooling curve), whereas curvesC1 through C3 represent three target cooling curves obtained by varyingthe cooling rates of the baseline cooling curve. Glass was drawn in afusion down draw process using the four cooling curves. Additionally,compaction was both predicted using equation (2) and measured.

Examination of curves C1-C3 reveal that while cooling curves C1 and C2closely follow the behavior of baseline curve B, cooling curve C3exhibits a greater slowing of the cooling rate in the region of about700° C. (commensurate with the indications of FIG. 4 ), and whichcomparatively slower cooling rate extends for a greater distancerelative to the root of the forming body than the other cooling curves.FIG. 6 graphically depicts (via box plots) measured compaction valuesfor each of the cooling curves of FIG. 5 , and indicates the number ofmeasured samples for each cooling curve. Compaction for each coolingcurve was also predicted using equation (2). The results are presentedin Table 1 below, where the predicted (modeled) compaction values andthe measured compaction values are provided in units of ppm.

TABLE 1 Condition Model Results Measured Base 20.60 20.6 C1 20.24 20.2C2 20.10 20.3 C3 19.77 19.6

Table 1 shows excellent agreement between the modeled (predicted)compaction values. The data also show the cooling curve C3 results in afull 1 ppm improvement in measured compaction values compared to resultsfrom the baseline cooling curve.

Referring back to FIG. 5 , it should be further noted that the targetcooling curves C1-C3 may also take into account considerations otherthan compaction. For example, adjusting a cooling curve for compactionmay affect other attributes of the glass ribbon, such as flatness of theribbon. Thus, while it is desirable to reduce compaction, compactionshould not be reduced such that other attributes suffer. As FIG. 5depicts, movement of the cooling curves upward (tilting upward), has theintended effect of reducing compaction, but may affect the physicalstability of the ribbon as the glass ribbon is drawn from the formingbody. Alternatively, tilting the cooling curves downward has the effectof reducing stress in the ribbon and therefore improving flatness, butat the expense of increased compaction. Accordingly, the improvement incompaction should be weighed against any detrimental effects on otherribbon attributes, such as residual stress and ribbon shape.

Thermal strain in the glass ribbon in a fusion forming processdetermines stress and shape in both the ribbon and in glass sheets cutfrom the ribbon, and can be calculated from a viscoelastic model. Suchmodels can be used to develop a metric useful for evaluating stress inthe glass ribbon. Ideally, thermal stress in the glass ribbon should bezero, or a tensile stress, so that the glass ribbon exhibitssubstantially zero warping. However, such treatment of stress cannot beapplied universally across the entire width of the glass ribbon. Forexample, the glass ribbon typically comprises thickened lateral edgeportions, termed “beads”, owing largely to surface tension effectsduring the draw process. Consequently, stress resulting from the beadscan be individually modeled. A separate metric can be developed for theeffect of the bead on shape of the glass ribbon. Broadly then, a morecomplete development of a cooling curve taking into account othervariables and considerations can include development of both a glassviscoelastic material model and a parameterized thermal model for ribbontemperature. Weights can be assigned to the various components of theobjective (for example compaction) to be optimized, and an optimaltemperature field for the ribbon can be calculated by manipulating theparameters of the thermal model until the objective is minimized.Application of the calculated temperature field to an actual process isdone by manipulating heater power, air flow, water cooling, etc.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to embodiments of the presentdisclosure without departing from the spirit and scope of thedisclosure. Thus it is intended that the present disclosure cover suchmodifications and variations provided they come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A method of controlling compaction, comprising:a) measuring compaction values for a plurality of glass sheets cut froma plurality of glass ribbons, the glass ribbons formed with differentcooling rates at temperature increments of at least 10° C. in atemperature range equal to or greater than 450° C. and equal to or lessthan 900° C.; b) correlating the measured compaction values with thecooling rates of step a) to obtain a plurality of regressioncoefficients corresponding to a plurality of temperatures, wherein stepb) comprises a linear regression, wherein the linear regressioncomprises a system of linear equations of the form ${{\begin{bmatrix}k \\\vdots \\k\end{bmatrix} + {\begin{bmatrix}q_{1}^{1} & \ldots & q_{1}^{n} \\\vdots & \ddots & \vdots \\q_{i}^{1} & \ldots & q_{i}^{n}\end{bmatrix} \times \begin{bmatrix}b_{1} \\\vdots \\b_{n}\end{bmatrix}}} = \begin{bmatrix}C_{1} \\\vdots \\C_{i}\end{bmatrix}},$ where q represents cooling rate in ° C./second, brepresents regression coefficients, C represents compaction in parts permillion, i represents a total number of data sets, n represents a totalnumber of the regression coefficients, and k represents an intercept ofthe regression; c) selecting a predetermined cooling curve, thepredetermined cooling curve comprising a plurality of predeterminedcooling rates at the corresponding plurality of temperatures of step b);d) using the plurality of regression coefficients and the plurality ofpredetermined cooling rates to calculate a predicted compaction value,wherein the predicted compaction value is equal to${{Compaction} = {k + {\sum_{\,{n = 1}}^{\,{n = m}}\left( {b_{n} \times q_{n}} \right)}}},$where m represents a number of the temperature increments over atemperature range from 450° C. to 900° C.; e) modifying thepredetermined cooling rates to minimize the predicted compaction valueand obtain target cooling rates; f) drawing a subsequent glass ribbonusing the target cooling rates.
 2. The method according to claim 1,further comprising substituting the modified cooling rates for thepredetermined cooling rates and repeating steps d) and e).
 3. A methodof controlling compaction, comprising: a) drawing a glass ribbon from aforming body at a first draw rate; b) measuring temperature along acenterline of the glass ribbon at a plurality of distances from a bottomedge of the forming body; c) calculating cooling rates for the glassribbon at the plurality of distances based on the measured temperatures;d) measuring a compaction value of a glass sheet cut from the glassribbon; e) repeating steps a) through d) for a plurality of glassribbons drawn at a plurality of draw rates to obtain a plurality ofmeasured compaction values; f) correlating the plurality of measuredcompaction values with the cooling rates at the plurality of distancesfor the plurality of glass ribbons to obtain a plurality of regressioncoefficients corresponding to a plurality of temperatures, wherein stepf) comprises a linear regression, wherein the linear regressioncomprises a system of linear equations of the form ${{\begin{bmatrix}k \\\vdots \\k\end{bmatrix} + {\begin{bmatrix}q_{1}^{1} & \ldots & q_{1}^{n} \\\vdots & \ddots & \vdots \\q_{i}^{1} & \ldots & q_{i}^{n}\end{bmatrix} \times \begin{bmatrix}b_{1} \\\vdots \\b_{n}\end{bmatrix}}} = \begin{bmatrix}C_{1} \\\vdots \\C_{i}\end{bmatrix}},$ where q represents cooling rate in ° C./second, brepresents regression coefficients, C represents compaction in parts permillion, i represents a total number of data sets, n represents a totalnumber of the regression coefficients, and k represents an intercept ofthe regression; g) selecting a predetermined cooling curve, thepredetermined cooling curve comprising a predetermined cooling rate ateach corresponding temperature of the plurality of temperatures; h)using the plurality of regression coefficients and the plurality ofpredetermined cooling rates to obtain a predicted compaction value at apredetermined draw rate, wherein the predicted compaction value is equalto${{Compaction} = {k + {\sum_{\,{n = 1}}^{\,{n = m}}\left( {b_{n} \times q_{n}} \right)}}},$where m represents a number of the temperature increments over atemperature range from 450° C. to 900° C.; i) modifying thepredetermined cooling rates to minimize the predicted compaction valueat the predetermined draw rate and obtain target cooling rates; j)drawing a subsequent glass ribbon using the target cooling rates.
 4. Themethod according to claim 3, further comprising substituting themodified cooling rates of step i) for the predetermined cooling rates ofstep h), and repeating steps h) and i) to obtain new target coolingrates prior to step j).